1. Field of the Invention
The invention relates to cathode ray tubes (CRT) with magnetic deflection systems, more particularly to a linearity compensation method using a variable magnetic field strength linearity compensation apparatus for a CRT with a magnetic deflection system.
2. Description of the Related Art
There is a wide range of display adapters presently available so as to make computer monitors suitable for a variety of applications. Such display adapters, which have different horizontal scanning frequencies and screen resolutions, include:
1. A CGA display adapter which has a horizontal scanning frequency of 15.7 kHz and a resolution of 640 * 200 (number of picture elements x number of scan line).
2. An EGA display adapter which has a horizontal scanning frequency of 21.8 kHz and a resolution of 720 * 350.
3. A VGA display adapter which has a horizontal scanning frequency of 31.5 kHz and a resolution of 640 * 350, 640 * 400 or 640 * 480.
4. A VGA (8514) display adapter which has a horizontal scanning frequency of 35.5 kHz and a resolution of 1024 * 768.
5. A SUPER VGA display adapter which has a horizontal scanning frequency of 37.8 kHz and a resolution of 800 * 600.
6. A NON-INTERLACE (8514) VGA display adapter which has a horizontal scanning frequency of 48.9 kHz and a resolution of 1024 * 768.
Computers are usually provided with a multi-scanning type computer monitor so as to make them compatible with two or more display adapters having different scanning frequencies and resolutions.
In a conventional computer monitor with a magnetic deflection-type cathode ray tube (CRT), a linearity coil is usually connected in series with the horizontal deflection system of the CRT. Referring to FIG. 1(A), the horizontal deflection system (10) includes a deflection coil (11), a transistor (12) and a damper diode (13). Because of the internal resistances of the deflection coil (11), the transistor (12) and the damper diode (13), the current at the front portion of each scanning cycle is greater than the current at the rear portion of the scanning cycle. If no linearity coil is used, the characters on the left portion of the CRT will be larger than the characters on the right portion of the CRT.
FIGS. 1(B) to 1(F) illustrate the signal waveforms at different nodes of the horizontal deflection system (10) shown in FIG. 1(A). FIGS. 2(A) to 2(D) are equivalent circuits of the horizontal deflection system (10) under different periods of one scanning cycle. A horizontal driving signal (Vb), as shown in FIG. 1(B), is fed to the base terminal of the transistor (12). During the t0-t1 period of the driving signal (Vb), the damper diode (13) is in a reverse bias condition, while the transistor (12) is in a conducting state. The equivalent circuit of the horizontal deflection system (10) at this stage is shown in FIG. 2(A).
The transistor (12) is cut off during the t1-t2 period of the driving signal (Vb). That is, no current (i.sub.c) flows through the collector of the transistor (12), as shown in FIGS. 1(C) and 2(B). The voltage (V.sub.c) across a capacitor (C) increases until a peak voltage (V.sub.cp) is reached, as shown in FIG. 1(F). The capacitor (C) discharges during the t2-t3 period of the driving signal (Vb) via the deflection coil (11), as shown in FIGS. 1(F) and 2(C). During the t3-t4 period of the driving signal (Vb), full discharging of the capacitor (C) has been completed, and the damper diode (13) starts to conduct, as shown in FIG. 1(E). The equivalent circuit of the horizontal deflection system (10) at this stage is shown in FIG. 2(D).
From the foregoing discussion, it has been shown that the internal resistance of the horizontal deflection system (10) is approximately equal to (R1) during the t0-t1 period of the driving signal (Vb), wherein (R1) is equal to the sum of the internal resistances of the deflection coil (11) and the transistor (12), and is approximately equal to (R2) during the t3-t4 period of the driving signal (Vb), wherein (R2) is equal to the sum of the internal resistances of the deflection coil (11) and the damper diode (13).
FIGS. 3(A) and 3(B) are equivalent circuits of the horizontal deflection system (10) during the t0-t1 and t3-t4 periods of the driving signal (Vb) when redrawn to include the internal resistances (R1, R2). The voltage (V.sub.Ly) across the deflection coil (11) is equal to [E-i.sub.y R1] during the t0-t1 period of the driving signal (Vb), and is equal to [E+i.sub.y R2] during the t3-t4 period of the driving signal (Vb). Thus, regardless of the values of the internal resistances (R1, R2), the characters on the left portion of the CRT will be larger than the characters on the right portion of the CRT unless a linearity coil is installed. FIG. 3(C) illustrate plots of the current (i.sub.y) flowing through the deflection coil (11) during ideal and normal conditions. The equations [(E/L.sub.y) * t] and {(E/L.sub.y) * [t-(T.sub.s /2)]} correspond to the current (I.sub.y) during ideal conditions [that is, the internal resistance of the horizontal deflection system (10) is equal to zero]. The equations (E/R1) * {1-exp[-(t * R1/L.sub.y)]} and (E/R2) * (1-exp{-[t-(T.sub.S /2)]*R2/L.sub.y ]}) correspond to the current (i.sub.y) during normal conditions wherein the internal resistance of the horizontal deflection system (10) is taken into consideration. Note that during the t0-t1 period of the driving signal (Vb), the effect of internal resistance is to reduce the voltage across the deflection coil (11), while during the t3-t4 period of the driving signal (Vb), the effect of internal resistance is to increase the voltage across the deflection coil (11).
The purpose of the linearity coil is to compensate for linearity variations due to the internal resistance of the horizontal deflection system (10) of the CRT. The principle of the linearity coil is as follows:
Referring to FIG. 4(A), a conventional coil device (C) is shown to comprise a coil wound on an I-shaped ferrate core. FIG. 4(B) is a B-H curve for the magnetic circuit shown in FIG. 4(A). Note that a substantial increase in the flux density of the coil device (C) is not possible because of magnetic hysteresis. FIG. 4(C) is a plot of current (i.sub.y) vs. inductance (L) for the coil device (C). Note that the coil device (C) becomes more and more saturated as the absolute magnitude of the current (i.sub.y) increases.
Referring to FIG. 5(A), a stationary permanent magnet (A) of appropriate magnetic field strength is mounted on one end of the core of the coil device (C) shown in FIG. 4(A). A plot of current (i.sub.y) vs. inductance (L) for the magnetic circuit shown in FIG. 5(A) is shown in FIG. 5(B). Note that the effect of providing the magnet (A) on one end of the core is to shift the current (i.sub.y) vs. inductance (L) curve shown in FIG. 4(C) to the left of the vertical axis. The degree of shift depends upon the magnetic field strength of the magnet (A). From the current (i.sub.y) vs. inductance (L) curve shown in FIG. 5(B), it can be seen that the magnetic circuit becomes more and more saturated [that is, the value of the inductance (L) decreases] as the value of current (i.sub.y) increases in the positive direction. However, the value of the inductance (L) increases as the value of current (i.sub.y) increases in the negative direction. The magnetic circuit shown in FIG. 5(A) is a conventional linearity coil used to compensate for linearity variations due to the internal resistance of the horizontal deflection system (10) of the CRT.
One of the drawbacks of the above described linearity coil is that the inductance of the coil and the current flowing therethrough cannot be tuned so as to correspond with the horizontal scanning frequency.
FIG. 5(C) is an illustration of another conventional linearity coil. The linearity coil is substantially similar to that shown in FIG. 5(A). However, the permanent magnet (A') is not mounted on one end of the core of the coil device (C) but is instead rotatably mounted adjacent to the core so as to provide a magnetic field which is adjustably combined with the magnetic field of the coil device (C), thereby permitting adjustments in the permeability of the core and hence, the relationship between the inductance of the linearity coil and the current flow therethrough. The effect of rotating the permanent magnet (A') is to permit initial adjustments in the degree of shifting of the current (i.sub.y) vs. inductance (L) curve, shown in FIG. 4(C), so as to obtain the best linearity compensation effect.
The linearity coil shown in FIG. 5(C) is ideal for use in cathode ray tubes which have a fixed horizontal scanning frequency, and is inconvenient for use in multi-scanning monitors since the linearity coil has to be tuned each time the horizontal scanning frequency varies.
Referring once more to FIGS. 3(A), 3(B) and 3(C), the voltage signal (E) should correspond to the horizontal scanning frequency so as to obtain a fixed scan width. If the horizontal scanning frequency is variable, as is the case in multi-scanning computer monitors, the voltage signal (E) is correspondingly varied.
It should be noted that the effect of the internal resistance of the horizontal deflection system (10) on the linearity distortion of the CRT is more severe when the magnitude of the voltage signal (E) is relatively small and is less severe when the magnitude of the voltage signal (E) is relatively large. This illustrates why multi-scanning monitors require a linearity compensation apparatus that has a magnetic field strength which can be automatically adjusted for a wide range of scanning frequencies.
FIGS. 6(A) and 6(B) are schematic electrical circuit diagrams of conventional linearity compensation apparatuses used in multi-scanning monitors. FIG. 6(A) illustrates a two-stage linearity compensation apparatus. When the horizontal scanning frequency is in a lower frequency range (such as 30 kHz to 45 kHz), the relay (23) is turned off, thereby connecting a pair of linearity coils (21, 22). This results in the generation of a deflection coil current vs. inductance curve with a relatively large negative slope, thereby negating the severe effects of internal resistance when the voltage signal (E) is relatively low. When the horizontal scanning frequency is in a higher frequency range (such as 45 kHz to 60 kHz), the relay (23) is turned on, thereby shorting the linearity coil (22). A deflection coil current vs. inductance curve with a smaller negative slope is generated, thus negating the effects of internal resistance when the voltage signal (E) is relatively high.
FIG. 6(B) illustrates a three-stage type linearity compensation apparatus which includes three linearity coils (31, 32, 33) and a pair of relays (34, 35) which are tuned to different ranges of horizontal scanning frequency. The operation of the linearity compensation apparatus shown in FIG. 6(B) is substantially similar to that shown in FIG. 6(A) and will not be detailed further.
The main drawbacks of the conventional linearity compensation apparatuses shown in FIGS. 6(A) and 6(B), regardless of the number of linearity coils employed, are as follows:
1. The linearity compensation effects at scanning frequencies near the upper and lower limits of the predesigned frequency ranges are relatively poor.
2. The conventional linearity compensation apparatuses are relatively expensive since they employ relays (23, or 34, 35).
3. Although increasing the number of linearity coils provides better linearity compensation, the size, complexity and cost of the system is correspondingly increased.